This invention relates to improving characteristics of cement by subjecting it to dense-phase gaseous (very high pressure) or supercritical (fluid) carbon dioxide (CO.sub.2) to alter the morphology and/or chemistry of hardened portland, lime or pozzolanic cement paste and permit manipulation of its properties and behavior. The invention further relates to testing cement to determine the extent to which additives in the cement may resist the carbonation of the cement.
As disclosed in my copending, commonly owned U.S. patent application (Ser. No. 08/390,468, filed Jan. 27, 1995, for Cement Mixtures With Alkali-Intolerant Matter and Method of Making Same), the disclosure of which is incorporated herein and made part hereof by reference, cement carbonation may be used to neutralize alkalinity to permit incorporation of alkali-intolerant materials into the wet paste to make superior products. That application discloses to expose cement to low-pressure carbon dioxide.
Cement carbonation, wherein naturally-occurring carbon dioxide in the atmosphere gradually combines with the calcium hydroxide in the cement matrix to form calcium carbonate and water, is generally considered undesirable because concrete containing steel reinforcement relies upon high alkalinity to inhibit steel corrosion. As carbonation takes place over time, alkali is reduced and the prophylaxis the steel receives against corrosion is lessened. Eventually, the steel begins to corrode, thereby weakening the concrete. The stoichiometry of the carbonation reaction is: EQU Ca(OH).sub.2 +CO.sub.2 .fwdarw.CaCO.sub.2 +H.sub.2 O
In contrast, deliberate carbonation to purposely reduce hydroxide using gaseous CO.sub.2, as is disclosed in my above-referenced patent application, quickly and completely eliminates the alkalinity in hardened cement pastes, whether the paste is acting alone or as part of other materials such as concretes or composites. The only morphological change that is visibly apparent under scanning electron microscope (SEM) examination is the absence of ettringite and portlandite and the appearance of visible micro-crystals of calcite (often called "dog teeth") in what prior to carbonation was calcium-silicate-hydrate gel. Some change is noted in the micromorphology, but pores and capillaries are still discernible and relatively plentiful. I have discovered that a much greater visible change takes place when dense-phase or supercritical carbon dioxide is infused. The diversity of structures is reduced and a regular "rice-grain" morphology is now evident. Another change that is clearly apparent in both cases using powder X-ray diffraction (XRD) is that the portlandite and ettringite peaks are absent in the spectrographic signature. They have been replaced by a very strong calcite peak. An identical chemical change can be observed in powder XRD of cement pastes carbonated by means of supercritical CO.sub.2.
As noted above, during experimentation it was discovered that when a cement matrix is exposed to carbon dioxide in its supercritical state, massive, observable morphological changes occur. The result is a densified, simplified microstructure with fewer different types of crystals, and exhibiting fewer micro-pores and microcapillaries than is typical of cements carbonated by means of relatively low-pressure gaseous CO.sub.2 or cements which have not been deliberately carbonated at all. The flat, plate-like structures indicative of portlandite, and the fine, needle-like crystals of ettringite are absent. In their place are rounded, closely packed, siliceous crystals with a "rice grain" appearance, neatly aligned with one another and with few or no visible pores or capillaries.
It was further discovered that supercritical CO.sub.2, long recognized as a polar solvent, can, if desired, simultaneously be used to infuse the hardened cement matrix with materials dissolved or suspended in the supercritical CO.sub.2 to alter the properties and behavior of the hardened cement. In addition, because certain cement whose original wet mixes contained methyl cellulose polymer as an additive refused to carbonate, even when exposed to the extreme pressures and concentrations of CO.sub.2 in its supercritical state, the very process of forcing supercritical carbon dioxide into the matrix enabled one to determine to what extent a cement would ultimately carbonate, if it would do so at all. It also became evident that the methyl cellulose prevented cement carbonation since it was the only material not present in the other mix designs tested.
Finally, it was discovered that other materials with matrices similar to cement, particularly ceramics whose pore structure and density can be easily controlled during formulation and firing, can also be infused with materials transported by the supercritical CO.sub.2.
The advantages and results obtained with the present invention as discussed below are attainable with supercritical CO.sub.2, as above defined, and dense-phase CO.sub.2. Both of them readily flow into and through cement (unless specially treated to close its passages), particularly under the high pressure of supercritical CO.sub.2.
CO.sub.2 becomes supercritical when it reaches a temperature of at least 31.degree. C. and a pressure of at least 1071 psi. Further, supercritical CO.sub.2 retains its supercritical characteristics even if, thereafter, its temperature drops below the supercritical threshold so long as at least the threshold pressure is maintained. Dense-phase CO.sub.2 is not supercritical and does not simultaneously behave like a liquid and a gas because it has not reached a temperature of 31.degree. C. and a pressure of 1071 psi. Dense-phase CO.sub.2 is highly compressed gas; say of a pressure of 80 to 100 atmospheres or more but which has never reached a temperature of at least 31.degree. C. so that it does not have the characteristics typical of supercritical CO.sub.2. For purposes of the present invention, dense-phase CO.sub.2 behaves similar to supercritical CO.sub.2 with the exception that dense-phase CO.sub.2, unlike supercritical CO.sub.2, does not dissolve or suspend certain materials soluable or suspendable in supercritical CO.sub.2 as is discussed below. Thus, unless otherwise stated, "supercritical CO.sub.2 " as used herein, including the claims, is intended to and does collectively refer to supercritical CO.sub.2 and dense-phase CO.sub.2 except in those instances, including the claims, which address the solubility or suspendability of certain materials in supercritical CO.sub.2, when the term "supercritical CO.sub.2 " excludes "dense-phase CO.sub.2 ".